1. Field of the Invention
The present invention relates generally to semiconductor processing equipment and specifically to systems and methods for calibrating a temperature control system for a substrate processing chamber.
2. Description of the Related Art
High temperature processing chambers are used for depositing various material layers onto semiconductor substrates, such as silicon wafers. A substrate is placed on a substrate holder, such as a susceptor, within the processing chamber. Both the substrate and the substrate holder are heated to a desired temperature. In an exemplary substrate treatment step, two or more reactant gases are conveyed over the heated substrate, where they chemically react with one another to cause chemical vapor deposition (CVD) of the reactant materials onto the substrate. Throughout subsequent processes, such as depositions, doping, lithography, etching, etc., these layers are made into integrated circuits, producing millions to billions of integrated devices, depending on the substrate size and the circuits' complexities.
As these integrated devices continue to become smaller, precise control of the processes becomes increasingly important. Various process parameters are carefully controlled to promote a high quality of the deposited layers, and in particular well-controlled and uniform film growth. Growth rate, and thus thickness uniformity of the deposited film, is a function of the mass transport of the reactant species conveyed to the substrate and the reaction rate at the surface of the substrate. At high temperatures, sometimes referred to as the “mass-transport limited regime,” the film growth rate is primarily a function of the reactants' partial pressures, and small temperature changes have minimal effects on the growth rate. At low temperatures, sometimes referred to as the “kinetic regime,” the film growth rate, while dependent on many variables, is dominated by temperature. Thus, substrate temperature is usually a critical process parameter in the kinetic regime, and a small change in temperature can result in a significant change in deposition rate and an undesirable or non-uniform layer thickness. Accordingly, it is usually important to accurately control the substrate temperature during deposition.
In a cold wall, single substrate reactor, temperature control systems are used to modify the heat output of heating elements, such as radiant heat lamps, in response to temperature readings from temperature sensors that measure temperature within the processing chamber. The temperature sensors typically comprise thermocouples mounted around and below the substrate, or optical pyrometers that allow temperature to be determined by measuring the substrate's thermal radiation.
Optical pyrometers, carefully positioned in the processing chamber, can facilitate the determination of substrate temperature by measuring the light radiation emitted by the substrate. Pyrometers are sometimes preferred over thermocouples because they react faster to temperature changes. However, if direct or reflected light from elements other than the substrate, such as from heating lamps, reaches the pyrometer, then the light radiation emitted from the substrate is only a part of the total radiation that the pyrometer receives, causing inaccuracies in the temperature readings. In some commercial systems, a pyrometer temperature measurement from the substrate is used directly as feedback to the heating/temperature control system. In order to ensure that only radiation from the substrate reaches the pyrometer, these systems must make significant design compromises, such as shielding the pyrometer or adjusting the placement of various components. Furthermore, the relationship between the substrate temperature and emissivity changes in different temperature ranges. Thus, it is easiest to use pyrometers within specific, discrete temperature ranges, and other factors must be considered when reading temperatures over a very broad range. For these reasons, thermocouples are often more preferable than pyrometers as a means for measuring the temperature of the substrate.
One problem in substrate reactors that use thermocouples is thermocouple drift, which is the tendency of thermocouple outputs to shift (i.e., report temperature less accurately, typically with a bias up or down) over time, for example over the course of many runs spanning days, months, or years. Thermocouple drift can be caused by a variety of reasons, including degradation of the material from which the thermocouple is formed (e.g., platinum degradation or grain slippage), deterioration of the thermocouple junction or shape, devitrification of the quartz envelope of the thermocouple, and movement of the thermocouple. Thus, temperature control systems are often calibrated from time to time to improve temperature control of the substrate.
In addition to thermocouple drift, there are a variety of other factors that can lead to inaccuracies in substrate temperature control. First, if heat lamps are used to heat the wafer, each lamp's power output can vary over time. Second, some reactors include reflective surfaces (e.g., gold surfaces) that surround the reaction chamber and reflect radiation toward the substrate and susceptor, and these reflective surfaces can degrade over time. Third, the emissivity of the susceptor itself can change over time. Fourth, material deposits (e.g., silicon) on the susceptor (e.g., a graphite susceptor with a silicon carbide coating) can cause temperature shifts as well.
One way in which a reactor's temperature control system is calibrated involves the use of thickness monitor wafers or boron-implanted wafers that can be used and then subsequently examined to determine the thickness of a film deposited on the wafer surface. Typical usage of a thickness monitor wafer involves depositing a film thereon and measuring the deposited film thickness with an ellipsometer or other metrology. The measured film thickness provides an estimate of the wafer temperature during the deposition. Typical usage of a boron-implanted wafer involves positioning it in a hot chamber and then measuring the extent of the boron diffusion into the wafer surface. Since diffusion is a function of time and temperature, the measured diffusion over a given time provides an estimate of the wafer temperature during the diffusion. These methods are undesirable in terms of reactor downtime and wafer cost, partly because they involve removing the wafer from the chamber for measuring film thickness or extent of diffusion.
Another way in which a reactor's temperature control system is calibrated involves visually interpreting color variations of a deposited layer, and adjusting the thermocouples accordingly. As noted above, variations in temperature result in variations in reaction rate across a substrate surface, particularly in the kinetic regime. These reaction rate variations produce differences in deposited layer thickness. It is known that, for selected thicknesses, a chemically deposited layer exhibits color variations corresponding to the thickness variations that result from surface temperature variations. Moreover, the relationship between color and temperature is understood sufficiently to know roughly how much to adjust heat output of the heating devices for a given color variation. After a film is deposited, a technician visually interprets the color variations and makes appropriate adjustments to the temperature control system to compensate for temperature non-uniformities that would have caused the color variations. Further details of this method are disclosed in U.S. Pat. No. 6,126,744. While this method of calibrating a temperature control system is very useful, it is somewhat limited and arbitrary because it depends upon a human technician's visual interpretation of color.